Nanotechnology Spotlight

Graphene controls color of nano-sandwiches

(Nanowerk Spotlight) Technology in our lives is ever more based on miniaturized structures that deliver higher performance devices taking up a fraction of the space compared to several years ago. But seeing what is going on at these tiny length scales comparable to molecules is very hard. Normally light cannot be used since it is not focused tightly enough, limited by the optical wavelength which is much larger than the structures we want to observe.

New research suggests that tightly squeezing light into small gaps in metallic nanostructures now provides a way to circumvent this problem. One of their extraordinary optical properties is that selected colors of light are strongly concentrated into the gaps, giving an extreme sensitivity to changes in the surrounding environment or geometry of the gap. This strong sensitivity is the basis for sensing applications through measuring the color of the nanostructure in different environments. Moreover, gaps on the sub-nanometer length scale in metallic nanostructures are interesting for a more fundamental reason: on these scales, quantum mechanical effects that are generally negligible for optical properties can play an important role.

Forming stable gaps less than a nanometer long has been really challenging since chemical reactions, vibrations, or contamination can easily alter the delicate configuration. Our new research ("Controlling Sub-nanometer Gaps in Plasmonic Dimers Using Graphene") shows that one reproducible and stable way of creating sub-nanometer gaps between metallic components is to use the new material graphene as a spacer. Graphene is the thinnest possible crystal consisting of only one single layer of carbon atoms.

We create nano-sandwiches with a gold mirror on the bottom, layers of graphene in the middle, and a gold nanoparticle on top of the graphene. The ultra-thin graphene traps light in the sandwich between the gold surfaces where it is strongly intensified.

Moreover, when light hits the nanoparticle, electrons in the metal start to move around and oscillate. Such periodic motion of electrons, similar to a water wave sloshing in a swimming pool, is called a plasmon. The gold surface underneath acts as a mirror for the gold nanoparticle sitting on the graphene: this nanoparticle sees its reflection in the mirror and the combination works like a pair of nanoparticles with a gap defined by graphene. The interaction between electrons in both particle and image impede their sloshing which changes the optimal colors trapped in the gap.

For the ultra-thin gaps from graphene we see completely new resonant colors of light that are very specific to the arrangement of atoms near the gap. A second layer of graphene for example increases the separation between the gold surfaces from one to two carbon atoms and simultaneously changes all the resonant colors drastically - one of them disappears completely.

With light now strongly confined in this tiny gap we can observe truly at the nanoscale just by detecting the color of the system. An added bonus is that the strong light localization opens up new possibilities to enhance photodetectors using such nano-sandwiches. This is particularly interesting for companies seeking camera technologies that can be implemented on top of existing devices, for instance on display screens.

Coupling between gold nanoparticle and its mirror image produces different colors Particles on graphene are mainly red while they appear green on multilayers. (Image: NanoPhotonics Group, University of Cambridge)

Finally, the nano-sandwiches form a new tool for measuring how graphene conducts electrons through its layers. The technique is not restricted to graphene as a spacer and other emerging two-dimensional materials can be characterized in the same way. This conductivity perpendicular to a two-dimensional crystal is is of great interest since many optoelectronic devices rely on electrical contacting the individual layers.

Our current aims are to actively control the conductivity of the graphene layer by applying a voltage. The modified graphene properties would give reversible and fast color tuning of the nano-sandwich. A new generation of ultra-compact optical switches and sensors is then possible, following the current trend of miniaturizing technology applications further down to the nanoscale. But no smaller gap is now possible since we finally reached the size of a single atom spacer.

By Jan Mertens and Jeremy Baumberg, NanoPhotonics Centre, University of Cambridge